A KINEMATIC MODEL FOR THE FORMATION OF DUPLEX SYSTEMS WITH A PERFECTLY PLANAR ROOF THRUST

We present a cross-sectional kinematic forward model for the formation of duplexes with a perfectly planar roof thrust. The major assumption s are a constant dip and constant spacing of the ramps in the undeform ed state and sequential deformation in the direction of tectonic trans port, with equal displacement along each ramp. The model is based on a coordinate transformation that simulates flexural slip parallel to th e active fault surface. This causes angular parallel folds and keeps t he layer thickness constant, except in the forelimbs of the horses. At tempts by previous workers to simulate the formation of duplexes with a perfectly planar roof thrust, on the other hand, were based on the a ssumptions of constant bed thickness and bed length, or a different to pology of the axial planes delimiting the forelimbs of the horses, and resulted in corrugated roof thrusts. We show that it is not possible to form a flat roof duplex type and preserve the forelimb thickness of the horses under flexural slip parallel to the active fault. We descr ibe duplexes by three parameters which are the separation s between ra mps, the ramp length l, and the displacement u along the ramps. In a u /s vs l/s diagram, duplexes with a perfectly planar roof thrust, resul ting from numerical experiments with our kinematic algorithm, occupy s pecific families of straight lines. Our results are independent of the dip or internal geometry of the thrust ramps. (C) 1997 Elsevier Scien ce Ltd.